Hey I didn't really understand the part of adding a new full masked layer... What do you mean by full masked layer? And what does it do so you can paint without going over his arm and without removing the curves in his shirt?Other then that really cool tutorial!

I duplicated the layer (the whole painting is now two identical layers), then I made the top layer much brighter and added a layer mask and made it completely black. Then I just painted the skull with white on the mask layer.